Concentration of minerals



Aug. 14, 1956 2,758,714

C. A. HOLLINGSWORTH CONCENTRATION OF MINERALS Filed Aug. 25, 1954 ENTOR CLINTON A.HOLL|NGSWORTH ATTORNEYS United States Patent CONCENTRATION OF MINERALS Clinton A. Hollingsworth, Lakeland, Fla., assignor to Smith-Douglas Company, Incorporated, Norfolk, Va., a corporation of Virginia Application August 25, 1954, Serial No. 452,177

11 Claims. (Cl. 209-168) This invention relates to the concentration of minerals by pneumatic flotation, and has for its object the provision of an improved flotation apparatus.

The apparatus of the invention is of particular advantage in the concentration of friable minerals, such as phosphate rock, which tend to become further disintegrated and to produce deleterious amounts of slime during the flotation treatment, especially in machines of the agitation type. It is presently the common practice in the Florida phosphate field to concentrate phosphate rock in two stages of froth flotation. In the first stage, a deslimed flotation feed of the raw rock is conditioned with caustic soda, fuel oil and a fatty acid such as tall oil, and the conditioned feed is subjected to froth flotation where a phosphate product (commonly called a rougher concentrate) is floated and the underflow (largely silica) is discarded to waste. The rougher concentrate normally contains 8 to 20% of silica, and hence its phosphate grade (BPL, i. e. bone phosphate of lime) is too low to be of much practical value. Accordingly, the rougher concentrate is de-oiled by scrubbing with sulphuric acid followed by desliming. The de-oiled product (commonly called the amine cell feed) is subjected to the second stage of froth flotation in the presence of a strong cationic collecting agent, such as an amine, where silica is floated and discarded to waste. The underflow of the second stage of flotation is a high grade phosphate product, and is transferred to storage. In the phosphate field, the phosphate product is called the concentrate regardless of whether it is the floated product or the underflow, and hence the silica float is called tailings.

Phosphate ores are friable and tend to produce deleterious amounts of slime during flotation treatment, especially in machines of the agitation type. Slime has been found to be particularly objectionable in the amine circuit flotation, since the presence of slime necessitates an increased amount of the relatively costly amine reagent with an attendant loss of phosphatic material in the silica tailings in order to obtain the desired low silica content in the phosphate concentrate. In the aforementioned two-stage practice, considerable slimes are produced in the scrubbing of the rougher phosphate concentrate with sulphuric acid, and hence the de-oiled product is de-slimed before subjected to the second stage of froth flotation. But even then, the types of flotation apparatus presently used in the Florida phosphate field produce objectionable amounts of slime during the flotation treatment.

The invention aims to provide an improved flotation apparatus in which the body of mineral pulp undergoing flotation is effectively aerated with a minimum of agitation and hence with the production of little, if any, slimes. To this end, the apparatus comprises a relatively deep tank having a porous bottom with a hydraulic compartment therebeneath, and is provided with means for introducing water into the compartment under sufficient pres- 2 ,758,714 Patented Aug. 14, 1956 "ice sure to force the water through the porous bottom into a column of mineral pulp undergoing flotation in the tank. The porous bottom of the tank is of substantially the same cross-sectional area as that of the adjacent lower portion of the tank and the hydraulic compartment therebeneath. A plurality of groups (preferably three) of horizontally disposed air-diffusers are mounted within the tank in superposed and vertically spaced positions. Mineral pulp feeding means and froth overflow means are provided near the top of the tank, and pulp discharge means are provided near the bottom of the tank.

The foregoing and other novel features of the invention will be best understood from the following description taken in conjunction with the accompanying drawings, in which Fig. 1 is a top plan of a pneumatic flotation apparatus embodying the invention, with part of the perforated froth-subduing plate broken away,

Fig. 2 is a side elevation of the apparatus, mostly in section along the line 22 of Fig. 1,

Fig. 3 is an end elevation, and

Fig. 4 is an enlarged sectional view of an air-diffuser.

As previously stated, the apparatus of the invention is of special advantage in the froth flotation treatment of friable minerals like phosphate rock. The flotation equipment and practices heretofore customary in the Florida phosphate field result in the production of considerable slime during the flotation treatment itself, so that the deleterious effects of slime onthe flotation treatment are not avoided by slime removal prior to flotation. In the apparatus of the invention slime production during the flotation treatment is so minimized as to be practically unobjectionable, and a deslimed-mineral pulp can be treated with high flotation efficiency.

The apparatus of the invention is built in separate units or sections which are vertically assembled and secured together to form the complete apparatus. The apparatus illustrated in the drawings is built up of a top unit A, an intermediate unit B and a bottom unit C. The top and bottom units may advantageously be about 2 feet in depth and the depth of the intermediate unit may then advantageously be from 2 to 3 feet. As many intermediate units B may be assembled in the complete apparatus as required to give the apparatus the desired over-all depth of pulp column. Each unit is provided with at least one group of horizontally disposed air-diffusers 5, so that when the units are assembled the various groups of airdilfusers are superposed and vertically spaced.

Each unit comprises a metal frame 6 to which are bolted or otherwise appropriately secured side and end members 7 and 8, respectively. The side and end members may advantageously be transparent thermoplastic resin plates, such as Rohm and Haas Co.s Plexiglas, through Which the action of the air-diffusers and the condition of the pulp column can readily be observed. Where the side and end members are of metal or other opaque material, the side members are preferably provided with transparent windows at levels opposite the air-diffusers, as indicated by the dotted'lines 7 on Fig. 2. The juxtaposed ends of the frames 6 of adjacent units have peripheral flanges 9 for bolting or otherwise appropriately securing the units together.

The construction of the air-diffusers is shown in detail in Fig. 4. Each difluser comprises a flexible tubular member 10, such as a rubber hose or the like, having a multiplicity of fine perforations 11 uniformly distributed throughout its cylindrical Wall. These perforations may conveniently be made by piercing the. wall with the needle of a leather or saddle sewing machine. Each hose 10 extends through registering holes in the side walls 7 and is closed at one end by a metal plug 12 screw-threaded to the end ofa perforatedpipe 13. The other end of the hoseis closed 'byametal plug'1'4; through which the pipe 13 extends, and the plugs 12 and 14 are securely held against their respective ends of the hose by a nut 15 carried ona screw-threaded portion of thepipe 13. The pipes 13- ofea'ch group-of air-diffusers are connected to a common header I6, and each header is connected through avalve- 17 to acompressed air supply main 18.

The bottom unit 'C has an hydraulic compartment 20 at its lower end which communicateswith the main upper portion-of theunit through-a porous medium 21. While theporous medium-is shown inFig. 2 of the drawings as consisting of-a punched" metalplatecovered-with a bed of lead shot about 1 inch in depth, themedium may consist of porous tile, punched metal plate,- metal' screen, heavy canvas, and the like: The hydraulic compartment 24} is connected t'oa suitable sourceof'water through a pipe 22 having a suitable control valve (not shown).

One end wall 8 of the bottom unit C has a lower pulpdischarge opening 23 and an upper pulp discharge opening 24 communicatingwith a common pulp discharge chute 25. The discharge opening 23 is just above the POIGUSTHedlUmiZI while-the discharge opening 24 is near the top of the unit, and the horizontally disposed airditfusers are about midwaybetween the twoopenings with: one diffuserpositioned" in the discharge chute 25. The chute: 25" extendsacross the width of the end wall 8, and' is provided at itsbottom-w-ith apulp discharge pipe 26. The rate of water introduction into the hydraulic compartment 20 is soproportioned with respect to the openings 23 and 2'4 and the pipe 26 that substantially all of the water introduced into the'compartment is discharged through the pipe 26-together with such water in the mineral pulp feed ofthe apparatus as is not carried away in the overflow from;

The. top unit A has a feed hopper 27 at the top of one end wall. and an adjustable froth overflow weir 28 at the top of the outwardly flaring opposite end wall 8. A perforated. plate 29"is horizontally positioned a short distance (normally aboutZ'inches) below the level of the weir 28, and extends from the inside bottom ofthe feed hopper 27 to. the firstf of three-vertically disposed (i. e. depending) andlhorizontallyrspaced transverse baffles 3t The-outward flare of the-end wallv 8 is such that the top of the unit A' is about tWiceLthelength of the bottom of theunit', and the horizontally spaced baflles 34) are positioned above the flared end wall.. A transverse perforated baflle 31 depends from about the'center of the horizontal perforated plate. 29,. with one air-difluser (nearest the discharge weir 28) ononev side of the baffle 31 and the H other air-diflusers of'unit A. on the other, or feed hopper, side of the bafile 31..

The intermediate unit B. is provided with a vertical chamber or. channel.32.along.thatrend.wall adjacent the flared end wall 8' of the top unit. The channel 32communicates at its top-with-the. top unit A and at its bottom with a similar but shorter channel 33' atthe upper end of the bottom unit C. The channel 33 communicates through an opening 34 with the. unit C at a level slightly above. that of the group of air-diffusers of the unit, and thev channel 32 communicates through an opening 35- with the unit. B at a level slightly above that of the group of air-diffusers in the unit. The channels 32. and 33areoflthesame-width as their respective units, andareprovided withtransverse dampers 36 and 37, respectively, for regulating: therpulp circulation. The dampers are adapted to. be-adjusted by a removable Wrench 38,. shown in. dottedlines in Fig. 2.

In operating the apparatus of the invention illustrated in the drawings, an. aqueous mineral pulp (preferably deslimed) is fed through the hopper 27 into a relatively deep column or'body of pulp undergoing froth flotation treatment. The flotation reagent (or reagents) may be mixed with the pulp beforeoras it is introduced into the hopper, or. all or. partofthe reagent. may be intro.- duced at one or more levels in the pulp column. Where itsproximity to the air diffusers.

the mineral pulp has been well deslimed, it is preferable to addthe reagent with the feed, but when thefeed contains any slime, some or all of the reagent should be introduced at one or more lower levels of the pulp column. Aeration takes place at the three (or more) different levels at which the groups of air-diffusers are positioned. Aeration is extensive in sectional area and gentle and uniform in character, due to the construction of the diffusers. The pipe 13' is of relatively small internal diameter, say one-fourth inch and its perforations are relatively large, while the flexible tubular member 10' is of substantially larger diameter, say one inch,, and its perforations are very fine, so that the interior of the tubular member is always filled with a substantial volume-of compressed air of uniform pressure throughout itslength Thus, at the level of each group of air-diffusers, a countless number of extremely fine, uniform and gentle streams of air are discharged into the pulp column from the entire cylindrical surface of the tubular member to'provid'e a dense mass of gentle and uniform rising. ai'r' streams or bubbles over the entire cross-sectional areaof-the pulp column.

Should some slime be included in the mineral pulp feed, it is removed inthe mineral frothat the top'of the pulp column, so that the mineral particles throughout the remaining depth-of the pulp column include little, if any, slime particles. By introducing the flotation agent or agents at or below the center level of the pulp column, the reagents pass upwardly and solid particles tend to sink downwardly, sothat the effective reagent concentration progressively increases downwardly where the pulp column is most free ofslime: By such removal of slime at the top of the column and countercurrent flow in the pulp column of reagent and mineral particles, the adverse affects of slime are greatly minimized.

Bubbles from the rising air streams attachthemselves to those mineral particles selectivelyconditioned' by the flotation reagent to float, and carry suchparticles into a layer or column of froth overlying the surface of the pulp body. For convenience, such conditioned mineral particles are herein called float particles while the otherparticles in the mineral pulp arecalled sinlc particles. Thus, in the first stageof the aforementioned two-stagefroth flotation concentration of phosphate rock; the float particles are phosphate and the sinkparticles are mostly silica, while in the second stage' or amine circuit thefloat particles aresilica and the sink particles are phosphate.

The perforated plate 25 is at approximately the liquid level of the pulp column; and serves-to minimize surface turbulence so that the overlying layer of' froth is relatively quiescent. As the froth passes longitudinally beyondthe' active influence of the top airdiffusers toward the overflow weir 23, it. moves over avrelatively quiescent body of pulp (adjacent and in direct communication with the pulp column) where opportunity is afforded fon sink particles to drop out of thefroth layer. Thus,- in eflect', there are in the top unit A an active aerated zone and an adjacent quiescent settling zone. The single air diffuser in the latter zone provides aeration for returning to the froth layer any float particles in" the immediate vicinity of the boundary between the two zones. The depending transverse baflies 30 and 31 divert descending sink particles away from the active aerated zone ofthe pulp column and generally in the direction ofthe open top of the channel 32. The baflle 31 extendsmore deeply into the pulp column than do the baflles- 30; because of The depending baflies- 36 and 31 further tend to minimize circulation of the pulp body directly below the froth layer, and'thus induce aquiescence in'the pulp body thatisbeneficial inguiding the'sink particles (dropped from the frothlayery' toward the open top of the channel 32..

There is a natural downward circulation of: pulp in the channels 32 and 33,.due tothe upwardimpetus that is given the pulp column: in the units: B and C by the; rising air streams. fromthe diffusers 5 in those. units. By adjustment of the dampers 36 and 37, any desired portion of the downwardly circulating pulp can be diverted from the channels 32 and 33 through the openings 35 and 34 into the units B and C, respectively. Thus, the channels provide means for short-circuiting sink particles to a lower level of the pulp column, and for returning any float particles to the pulp column at levels of active aeration. In this manner, as well as in the characteristic arrangement of the air-diffusers at several different levels in a relatively deep pulp column, float particles are given several opportunities of attaching themselves to rising air bubbles thereby minimizing the amount of such particles included in the underflow.

The introduction of a controlled amount of water from the hydraulic compartment 20 into the lower portion of the pulp column is a characteristic feature of the apparatus. The water so introduced into the pulp column is conveniently called the hydraulic water. It promotes circulation within the pulp column and prolongs the period of suspension of float particles and gives them better, longer and more frequent opportunity for attachment to rising air bubbles. The hydraulic water also prevents sanding-up of the apparatus, since the pulp density of the underflow discharge is lowered by the water entering the pulp column in the region of the underflow discharge. The pulp density of the feed to the apparatus is generally Within the range of 30 to 75% solids and is higher than the pulp density of the underflow discharge, which is generally within the range of 20 to 40% solids. From 25 to 50% of the water of the pulp column is introduced with the feed, and the remaining percent (i. e. from 75 to 50%) is introduced as hydraulic water. The volume of hydraulic water introduced into the unit C (per unit of time) is at least substantially equivalent to the volume of water in the underflow discharge from the pipe 26 in the same time period.

As sink particles descend into the unit C, some will be carried by the pulp stream through the upper opening 24 into the discharge chute 25. The single air-diffuser near the top of the chute offers a further and last opportunity for any float particles to be carried upward by attachment to rising air bubbles. Similarly, the remainder of the lowermost air-dilfusers in the unit C promote final cleaning of the pulp stream of float particles before discharge of the pulp through the bottom opening 23 into the chute 25.

Withdrawing with the underflow discharge of substantially the same volume of water as introduced into the unit C through its porous bottom decreases the downward velocity of pulp flow and thus provides and insures a relatively quiescent column of mineral pulp above the unit C. This relatively quiescent column of pulp provides better and longer contact of float particles with air bubbles, and thus promotes separation of float and sink particles and minimizes slime production. In this quiescent column of pulp there are only gentle circulatory motions resulting from the effects of the rising air streams and the channels 32 and 33, and hence little, if any, tendency to create slime.

The following are typical results obtained in the apparatus of the invention with the amine circuit feed of the aforementioned two-stage practice of concentrating phosphate rock. The pulp column of the flotation apparatus was about 6 feet in depth and about 14 inches by 24 inches in horizontal cross section. The amino cell feed analyzed 69.28% BPL and 10.14% silica determined as insoluble matter (insol.) as is common in the phosphate industry. The flotation reagent was a mixture of 55% di-isobutyl carbinol (D. I. B. C.) and 45% of a commercial cationic collecting agent of the amine type (specifically Armour and Co.s Annoflote S, consisting of about 70% free fatty amine and about 30% nitrile). The phosphate concentrate analyzed 74.60% BPL and 3.5% insol., and the percent recovery of BPL was 99.4. The (silica) tails analyzed 5.18% BPL. Because of slime formation during the flotation operation, the mixture of the amine flotation agent with a frothin'g agent of the alcohol type (such as D. I. B. C.) has not proven satisfactory and hence the prevailing practice has been to mix the amine agent with kerosene, by which practice the composite result obtained during 1953 in a commercial phosphate flotation plant was a phosphate concentrate averaging 76.48% BPL and 4.42 insol., a silica tails averaging 45.97% BPL with an average BPL recovery of 88.6% from an amine cell feed averaging 71.13% BPL and 10.12% insol.

In another series of tests carried out in a phosphate flotation plant under commercial operating conditions the following composite results were obtained with the apparatus of the invention. The flotation reagent was a mixture of equal parts of Armoflote S and D. I. B. C.:

Feed Concentrate B P L B P L in Tails Recovery B P L Insol. B P L Insol.

percent percent percent percent percent percent 73. 62 8. 35 3. 26 l1. 18 99.

In still another series of tests, the rougher cell raw feed (of the aforementioned two-stage practice) was treated directly in the apparatus of the invention with the following results, the flotation reagent being a mixture of equal parts of Armoflote S and D. I. B. 0:

Feed Concentrate B P L B P L in Tails Recovery B P L Insol. B P L Insol.

percent percent percent percent percent percent 30. 94 59. 62 68. 77 11. 6G 8. 63 82. 5

Another series of tests with rougher cell raw feed gave the following results, the flotation reagent being the reaction product of sulphuric acid (4.76% by weight) and a mixture of Armoflote S (19.05%) and crude turpentine (76.19%), such as described and claimed in the copending patent application of Jordan L. Wester and myself, Ser. No. 443,692, filed July 15, 1954 and now These results are comparable with those now obtained in the first or rougher (phosphate float) stage of the aforementioned two-stage practice, and show that the troublesome intermediate step (with its creation of much slime) of deoiling the phosphate concentrate can be dispensed with. The rougher phosphate concentrate obtained in these tests is comparable to the aforementioned amine cell feed and is readily cleaned (without intermediate treatment) in a second flotation operation conducted in accordance with the principles of the invention. While the second series of tests gave a rougher phosphate concentrate of relatively high insol. content (20.40%), this is within the range considered practical for a rougher concentrate (i. e. less than 25% insol.)

The construction of the relatively deep columnar tank of mechanically separate units not only makes readily possible changes in the depth of the tank to meet different flotation requirements, but facilitates servicing of the apparatus. Usually, the tank will accommodate a pulp column of at least six feet in depth. The air-diffusers are of such construction that air is uniformly introduced into the pulp column in a very finely disseminated state over'the entire cross-sectional area of the pulp column. Such introduction into the pulp column at two or more different levels of a dense mass of gentle and uniform air streams or bubbles insures most effective aeration, and gives float particles repeated opportunities for attachment to the air bubbles rising over the entire cross-- sectional area and throughout the entire depth of the relatively deep pulp column. The air introduced by each group of diffusers is separately controlled by its valve 17, and compensation can thereby be made for different depths of submergence in the pulp column. The action of the different groups of. air-diffusers may be watched and checked through the transparent walls of the apparatus, or through the transparent windows 7. The provision near the top of the pulp column of a relatively quiescent body of unaeratedpulp' in communication with the adjacent actively aerated pulp permits cleaning of the overflow froth as it passes over the unaerated pulp body on its way to the froth overflow lip. The unaerated and relatively quiescent pulp body is of substantial crosssectional area and depth, and solid particles freely settling therethrough are diverted by the baffles 30 and 31 into the' open top of the channel 32, and thence through one or both of the openings 35 and 34 into the actively aerated pulp column for retreatment.

Normally, the froth column or bed will be from 1 to inches in depth. The depth of froth depends to a large extent on the type of flotation reagents employed, some yielding a voluminous froth while others give very little froth. The depth of froth can be controlled to some extent by the use of different frothing agents, surface active agents, depressants etc. The perforated froth-subduing plate 29 is normally about 2 inches below the level of the adjustable weir 28. The pulp (i. e. liquid). level may be raised or lowered, in relation to the perforated plate 29, by adjusting the number of filler-bars that make up the adjustable weir-28. Thus, the pulp level should be: lowered for a shallow froth bed and raised for a deep froth bed.

A further advantage of the invention is the ability to introduce the flotation reagents at any depth in the pulp column. Thus, the mineral pulp feed may be conto the pulp column at any desired level. Since the discharge of Water in the pulp underflow is at least equal to the amount of hydraulic water introduced into the bottom of the'pulp column, the mineral'particles in the pulp feed to the hopper 27 are contacted with a higher concentration of reagents, irrespective of where introduced, than would be the case if all of the Water were introduced in the mineral pulp feed to the hopper 27. This insures a more effective conditioning of the mineral particles with the flotation reagents.

I' claim:

1. A flotation apparatus comprising a tank of substantial depth having near the top thereof pulp feeding means and froth overflow means, pulp discharge means near the bottom of said tank, a hydraulic compartment communieating through a porous medium with the bottom of said tank, the compartment, porous medium and bottom of the tank being of substantially the same cross-sectional area, means for introducing water into said compartment under suflicient pressure to force water through said porous medium into said tank, and a plurality of groups of horizontally disposed air-diffusers mounted within the tank in superposed and vertically spaced positions.

2. A flotation apparatus comprising a relatively deep tank having a porous bottom, a hydraulic compartment beneath said porous bottom, means adapted to introduce water into said compartment and to force the water through said porous bottom into said tank, a plurality of superposed and. vertically spaced groups of air-diffusers with the diffusers in each group positioned in approximately the samehorizontal plane, means positioned above said porous bottomand below the lowermostgroup of airdifrusers for withdrawing pulp from the tank, froth overflow means positioned along one side of said tank near the topthereof, and means on the opposite side of said tank and near the top thereof for feeding pulp to the tank.

3. A flotation apparatus comprisingat least three mechanically separate units superposed and secured together to form arelatively deep columnar tank, a group of horizontally disposed air-diffusers mounted in each unit, the lowermost of said units having a hydraulic compartment communicating through a porous medium with the bottom of the columnar tank, means for introducing water into said compartment under sufficientpressure to force water through said porous medium into the bottom of the columnar tank, and pulp discharge means operatively associated with said lowermostunit, the uppermost of said units having near the-top thereof pulp feeding means and froth overflow means and further havinga flaring end wall adjacent the frothoverflow means and beyond the active influence-of the air-diffusers of the unit and providing the unit with an unaerated portion of substantial depth and cross-sectionalareaadjacent said froth overflow means.

4. A- flotation apparatus comprising atank of substantial depth, pulp discharge meansnear the bottom of said tank, ahydraulic compartment communicating through a porous medium with the bottom of said tank, means for introducing waterinto said compartment, a plurality of groups of horizontally disposed air-diffusers mounted within the tank and superposed in vertically spaced positions, said tank having near the top thereof pulp feeding means and froth overflow means and further having a flaring end wall adjacentthe froth overflow means and beyond the active influence of theair-diffusers of the unit and providing the upper part of the tank with an unaerated portion of substantial depth and cross-sectional area between said froth overflow meansand that portion of the tank immediately above the uppermost group of airdiffusers.

5. A flotation apparatus according to claim 4, further characterized by the provision of a perforated plate horizontally positionedat approximately the level of said froth overflow means and overlying the uppermost group of air-diffusers and overlying only part of the unaerated portion near the top of the tank.

6. A- flotation apparatus according. to claim 5, further characterized by the provision of a baffle positioned betweensaid pulp' feeding means and said froth overflow meansand dependingfrom saidperforated plate to a depth belowsaid uppermost group-of air-diffuserswith most of the air diffusers on the pulp feeding side of the baffle and at least one air diffuser on the froth overflow side of the baffle.

7. A flotation apparatus according. to claim 4,.further characterized by the'provision-of a vertical channel alongside one end' of the tank with the upper open end of the channel directly below said unaerated portion and with the lower open end of the channel-communicating with the tank at a-substantially lower level.

8. A- flotation apparatus according to claim 7, further characterized by the provision of a plurality of horizontally spaced baffles depending from approximately the level of said froth overflow means into the unaerated portion near the-top'of the tank.

9. A flotation apparatus comprising a relatively deep tank having near thetopthereof pulp feeding'means and froth overflow means, a hydraulic compartment communicatingthrough a porous mediumwith the bottom of said tank, means for introducin'g'water' into said compartment under suflicient pressure to force water through said porous medium into said tank, a plurality of groups of horizontally disposed air-diffusers mounted within the tank in superposed and'vertic'ally spaced positions, and a pulp discharge chute a'longsidethe' lowerpo'rtion of said 9 tank in communication with the interior of the tank at two different levels above and below, respectively, the level of the lowermost horizontally disposed group of air-diffusers.

10. A flotation apparatus according to claim 9, further characterized in that at least one air-diffuser of said lowermost group is positioned in said discharge chute while most of the air-diffusers of the group are positioned within the tank and over said porous medium.

11. A flotation apparatus comprising at least three mechanically separate units superposed and secured to gether to form a relatively deep columnar tank, a group of horizontally disposed air-difiusers mounted in each unit, the lowermost of said units having a hydraulic compartment communicating through a porous medium with the bottom of the columnar tank, means for introducing water into said compartment under sufficient pressure to force water through said porous medium into the bottom of the columnar tank, pulp discharge means operatively associated with said lowermost unit, the uppermost of said units having near the top thereof pulp feeding means and froth overflow means and further having a flaring end wall adjacent the froth overflow means and beyond the active influence of the air-diifusers of the unit and providing the unit with an unaerated portion of substantial depth and cross-sectional area between said froth overflow means and that portion of the unit in which the air-diflusers are mounted, and a vertical channel alongside one end of the unit positioned immediately below the uppermost unit and outside the influence of the air-diffusers in the lower unit, the upper open end of said channel being directly below the unaerated portion of the uppermost unit and the lower open end of the channel communicating with a unit therebelow.

References Cited in the file of this patent UNITED STATES PATENTS 1,167,835 Norris Jan. 11, 1916 1,920,719 StiCh Aug. 1, 1933 2,176,107 Smith Oct. 17, 1939 

1. A FLOTATION APPARATUS COMPRISING A TANK OF SUBSTANTIAL DEPTH HAVING NEAR THE TOP THEREOF PULP FEEDING MEANS AND FROTH OVERFLOW MEANS, PULP DISCHARGE MEANS NEAR THE BOTTOM OF SAID TANK, A HYDRAULIC COMPARTMENT COMMUNICATING THROUGH A POROUS MEDIUM WITH THE BOTTOM OF SAID TANK, THE COMPARTMENT, POROUS MEDIUM AND BOTTOM OF THE TANK BEING OF SUBSTANTIALLY THE SAME CROSS-SECTIONAL AREA, MEANS FOR INTRODUCING WATER INTO SAID COMPARTMENT UNDER SUFFICIENT PRESSURE TO FORCE WATER THROUGH SAID POROUS MEDIUM INTO SAID TANK, AND A PLURALITY OF GROUPS OF HORIZONTALY DISPOSED AIR-DIFFUSERS MOUNTED WITHIN THE TANK IN SUPERPOSED AND VERTICALLY SPACED POSITIONS. 